Pregnancy Due Date Calculator
Estimate your due date, how many weeks along you are and your current trimester.
How to use the Pregnancy Due Date Calculator
- Pick the first day of your last menstrual period (LMP).
- The calculator adds 280 days (40 weeks) to estimate your due date.
- See how many weeks along you are and which trimester you’re in.
Due date = first day of last period + 280 days (Naegele’s rule)
About the Pregnancy Due Date Calculator
This calculator uses Naegele’s rule — the method midwives and doctors use — to estimate your due date by adding 280 days (40 weeks) to the first day of your last menstrual period. It also shows roughly how many weeks along you are today and your current trimester.
Pregnancy is counted from the last period, not from conception, which is why you’re already “two weeks pregnant” at the moment of conception. Only about 1 baby in 20 arrives on the estimated date; most come within the two weeks either side.
The estimate assumes a regular 28-day cycle. If your cycles are longer or shorter, or you know your conception date, a dating scan from your care provider will be more accurate.
Frequently asked questions
How accurate is the due date?
It is an estimate. Only around 5% of babies arrive on the exact date — most are born within two weeks either side. An early ultrasound gives the most accurate dating.
What if my cycle isn’t 28 days?
Naegele’s rule assumes a 28-day cycle. Longer cycles push the due date later and shorter cycles earlier; your provider can adjust for this.
Why count from my last period?
The last period is a date most people can identify, while the exact day of conception is usually unknown. That’s why clinical dating starts there.